I'm having the exact same problem, but I'm getting over 2,000 warnings. It says "Sitemap contains URL's which are blocked by robots.txt" and it refers to this: wp-content/uploads, which is not blocked by the robots.txt file.
Here's what I have on my robots.txt file:
User-agent: *
Disallow: /feed/
Disallow: /cgi-bin/
Disallow: /wp-admin/
Disallow: /wp-includes/
Disallow: /trackback/
Disallow: /xmlrpc.php
Disallow: ?wptheme=
Disallow: /unpublished/
Allow: /tag/mint/
Allow: /tag/feed/
Allow: /wp-content/online/
Allow: /wp-content/uploads/
Sitemap: http://www.singleinstilettos.com/sitemap_index.xml
Sitemap: http://www.singleinstilettos.com/post-sitemap.xml
Sitemap: http://www.singleinstilettos.com/page-sitemap.xml
Sitemap: http://www.singleinstilettos.com/attachment-sitemap.xml
User-agent: ia_archiver
Disallow: /
Can someone give both of us some advice on how to fix? Thanks!!